When a metal or metal alloy such as steel is cast in a mould that is open in both ends in the casting direction by means of a continuous or semi-continuous process, a melt is delivered to the mould as a free tapping jet (open casting) or via a casting tube (closed casting). As the melt passes through the mould a strand is formed by the cooled melt. Before the strand leaves the mould at least a self-supporting surface layer has been formed on the strand. An uncontrolled flow of metal in the non-solidified portions of the strand results in problems with regard to quality as well as the production itself.
The flow of melted metal in the non-solidified portions of a strand can be controlled by means of a two-phase or plural-phase stirrer by means of which a rotating magnetic field can be forced to act on the melt in these portions of the strand, as electric currents are induced in the melt. Thereby, due to a common effect of the magnetic field and the currents, stirring forces are created in the melt. A typical stirrer can be compared to the stator of an electric motor, where the melt forms the inner rotor. However, the losses are larger than by a conventional motor, partly depending on the gap that, for the majority of the stirrer constructions, is present between the cores and the melt.
In order to obtain the required string force, and accordingly satisfying metallurgical results, the movements of the melt must be controlled and distributed. Therefore, there are requirements on the properties and the distribution of the field strength of the magnetic field that is applied to the melt, as well as on the current intensity and the current density of the induced currents.
The requirements may differ between different casting processes, such as open and closed casting.
By closed casting, that is, when the melt is delivered to the mould through a casting tube that outlets in the melt below the so-called meniscus, there is required a stirring in the non-solidified portions, said sting being of enough strength to guarantee the requested metallurgical result as to the casting structure. However, stirring in the meniscus area should be avoided because such stirring could result in slag and casting powder on the meniscus surface being pulled down into the melt and then generating inclusions therein as the melt solidifies.
By open casting, that is, when melt is delivered to the mould from a container or a box or the like, as a free jet, it is required that the flow velocity at the meniscus is sufficiently high in order to obtain the requested metallurgical results. A sufficiently strong flow at the meniscus is obtained by giving the applied magnetic flew-flux a sufficiently high strength at the meniscus.